Usage

<?php register_taxonomy( $taxonomy, $object_type, $args ); ?>

Use the init action to call this function. Calling it outside of an action can lead to troubles. See #15568 for details.

Better be safe than sorry when registering custom taxonomies for custom post types. Use register_taxonomy_for_object_type() right after the function to interconnect them. Else you could run into minetraps where the post type isn't attached inside filter callback that run during parse_request or pre_get_posts.

Parameters

$taxonomy

(string) (required) The name of the taxonomy. Name should only contain lowercase letters and the underscore character, and not be more than 32 characters long (database structure restriction).

{custom_post_type} - Custom Post Type names must be all in lower-case and without any spaces.

null - Setting explicitly to null registers the taxonomy but doesn't associate it with any objects, so it won't be directly available within the Admin UI. You will need to manually register it using the 'taxonomy' parameter (passed through $args) when registering a custom post_type (see register_post_type()), or using register_taxonomy_for_object_type().

Arguments

(string) (optional) A plural descriptive name for the taxonomy marked for translation.

Default: overridden by $labels->name

labels

(array) (optional) labels - An array of labels for this taxonomy. By default tag labels are used for non-hierarchical types and category labels for hierarchical ones.

Default: if empty, name is set to label value, and singular_name is set to name value

'name' - general name for the taxonomy, usually plural. The same as and overridden by $tax->label. Default is _x( 'Post Tags', 'taxonomy general name' ) or _x( 'Categories', 'taxonomy general name' ). When internationalizing this string, please use a gettext context matching your post type. Example: _x('Writers', 'taxonomy general name');

'popular_items' - the popular items text. This string is not used on hierarchical taxonomies. Default is __( 'Popular Tags' ) or null

'separate_items_with_commas' - the separate item with commas text used in the taxonomy meta box. This string is not used on hierarchical taxonomies. Default is __( 'Separate tags with commas' ), or null

'add_or_remove_items' - the add or remove items text and used in the meta box when JavaScript is disabled. This string is not used on hierarchical taxonomies. Default is __( 'Add or remove tags' ) or null

'choose_from_most_used' - the choose from most used text used in the taxonomy meta box. This string is not used on hierarchical taxonomies. Default is __( 'Choose from the most used tags' ) or null

'not_found' (3.6+) - the text displayed via clicking 'Choose from the most used tags' in the taxonomy meta box when no tags are available. This string is not used on hierarchical taxonomies. Default is __( 'No tags found.' ) or null

(boolean) (optional) Whether to generate a default UI for managing this taxonomy.

Default: if not set, defaults to value of public argument. As of 3.5, setting this to false for attachment taxonomies will hide the UI.

show_in_nav_menus

(boolean) (optional) true makes this taxonomy available for selection in navigation menus.

Default: if not set, defaults to value of public argument

show_tagcloud

(boolean) (optional) Whether to allow the Tag Cloud widget to use this taxonomy.

Default: if not set, defaults to value of show_ui argument

meta_box_cb

(callback) (optional) Provide a callback function name for the meta box display. (Available since 3.8)

Default: null

Note: Defaults to the categories meta box (post_categories_meta_box() in meta-boxes.php) for hierarchical taxonomies and the tags meta box (post_tags_meta_box()) for non-hierarchical taxonomies. No meta box is shown if set to false.

(boolean) (optional) Is this taxonomy hierarchical (have descendants) like categories or not hierarchical like tags.

Default: false

Note: Hierarchical taxonomies will have a list with checkboxes to select an existing category in the taxonomy admin box on the post edit page (like default post categories). Non-hierarchical taxonomies will just have an empty text field to type-in taxonomy terms to associate with the post (like default post tags).

update_count_callback

(string) (optional) A function name that will be called when the count of an associated $object_type, such as post, is updated. Works much like a hook.

Default: None - but see Note, below.

Note: While the default is '', when actually performing the count update in wp_update_term_count_now(), if the taxonomy is only attached to post types (as opposed to other WordPress objects, like user), the built-in _update_post_term_count() function will be used to count only published posts associated with that term, otherwise _update_generic_term_count() will be used instead, that does no such checking.

This is significant in the case of attachments. Because an attachment is a type of post, the default _update_post_term_count() will be used. However, this may be undesirable, because this will only count attachments that are actually attached to another post (like when you insert an image into a post). This means that attachments that you simply upload to WordPress using the Media Library, but do not actually attach to another post will not be counted. If your intention behind associating a taxonomy with attachments was to leverage the Media Library as a sort of Document Management solution, you are probably more interested in the counts of unattached Media items, than in those attached to posts. In this case, you should force the use of _update_generic_term_count() by setting '_update_generic_term_count' as the value for update_count_callback.

query_var

(boolean or string) (optional) False to disable the query_var, set as string to use custom query_var instead of default which is $taxonomy, the taxonomy's "name".

Default: $taxonomy

Note: The query_var is used for direct queries through WP_Query like new WP_Query(array('people'=>$person_name)) and URL queries like /?people=$person_name. Setting query_var to false will disable these methods, but you can still fetch posts with an explicit WP_Query taxonomy query like WP_Query(array('taxonomy'=>'people', 'term'=>$person_name)).

'ep_mask' - (Required for pretty permalinks) Assign an endpoint mask for this taxonomy - defaults to EP_NONE. If you do not specify the EP_MASK, pretty permalinks will not work. For more info see this Make WordPress Plugins summary of endpoints.

Note: You may need to flush the rewrite rules after changing this. You can do it manually by going to the Permalink Settings page and re-saving the rules -- you don't need to change them -- or by calling $wp_rewrite->flush_rules(). You should only flush the rules once after the taxonomy has been created, not every time the plugin/theme loads.

(boolean) (optional) Whether this taxonomy should remember the order in which terms are added to objects.

Default: None

_builtin

(boolean) (not for general use) Whether this taxonomy is a native or "built-in" taxonomy. Note: this Codex entry is for documentation - core developers recommend you don't use this when registering your own taxonomy

Default: false

Example

An example of registering a two taxonomies, genres and writers, for the post type called "book" (uses Version 3.1 arguments):

Note: You can define custom taxonomies in a themes's functions.php template file:

Note: If you want to ensure that your custom taxonomy behaves like a tag, you must add the option 'update_count_callback' => '_update_post_term_count'. Not doing so will result in multiple comma-separated items added at once being saved as a single value, not as separate values. This can cause undue stress when using get_the_term_list and other term display functions.

Example Private Taxonomy

If you do not want your taxonomy to be exposed publicly, you can use the 'public' and 'rewrite' parameters to suppress it. It will be available to use internally by your plugin or theme, but will not generate a url of it's own.

Reserved Terms

Avoiding the following reserved terms is particularly important if you are passing the term through the $_GET or $_POST array. Doing so can cause WordPress to respond with a 404 error without any other hint or explanation.